1. Field of the Invention
This invention relates to a thermal printer, with a thermal print head having a series of electrically drivable heating elements, with a microprocessor transferring data to be printed into a register associated with the thermal print head and with driving driver circuits connected to the heating elements, the register receiving the data in serial form.
2. Background Information
In known thermal printers, thermal print heads are used which include a series of electrically selectively drivable heating elements. Between the heating elements and a platen roll, the recording medium to be printed upon and, if the recording medium is not thermally active, the effective part of a thermally activatable ribbon, are arranged so that a current flowing through the heating elements results in dyeing (as a rule, blackening) of the recording medium. As known, for example, from DE 36 13 946 A1 or EP 501 707 A2, the data to be printed is forwarded to the thermal print heads via a serial data line. Thus, data is transmitted via only one line from a microprocessor arranged on the mother board to the print head, paralleled there by means of a (shift) register, one register corresponding to one print line. The data is used for driving the driver circuits of the heating elements.
In such an arrangement, the high expenditure of time involved in transmitting data serially is generally to be regarded as a disadvantage. Considering that thermal print heads built to today's design mostly comprise more than 1,000 heating elements and just as many register elements, the time lag caused by changing the data to a serial form by means of the microprocessor and transmitting it sequentially can be rather significant. Because the microprocessor is not available for other tasks during this time lag, such as for arithmetic operations or for the control of the platen roll, the microprocessor has to execute such tasks after the time lag. As a result, the maximum printing speed can be substantially reduced.
Further, in printers, the stepping motor driving the platen roll can be controlled (as known, for example, from JP 60-83864 A) in such a way that a control circuit supplies pulses to a stepping motor, whenever the number of pulses tends to correspond to the respective feed of the recording medium. Because, for reasons of cost and for the purpose of avoiding synchronization problems, the same microprocessor tends to control, as a rule, both the data transmission to the print head and the stepping motor of the platen roll, software routines can be conventionally used. As a result, corresponding pulses can be issued to the stepping motor, while blocking the microprocessor for other tasks during the feeding cycle of the recording medium.
Likewise, such a software-based control of the stepping motor can result in the disadvantage of a considerably reduced printing speed.